Dancing Slime
Musical taste is very personal. Some people will only listen to music of a certain genre, while others are more open to multiple genres. Oobleck, a very specific type of slimy mixture, has a very unique taste in music. It’s not concerned with the lyrics, genre, or the artist. Its only concern is the frequency, and when the right frequency is played, Oobleck can’t help but break out some serious dance moves! In this activity, you will explore the properties of sound waves, discover some of the unusual characteristics of Oobleck, and learn how the two interact to make Oobleck such a talented dancer!
Intro
Cornstarch
Cornstarch - 180 ml
Food colouring - 5 drops (optional)
1/4 cup
Measuring spoon - 1/4 cup
Warm water - 125 ml
Liquid measuring cup
Mixing bowl
Plastic or metal mixing spoon
AUX cord
Subwoofer/
Speaker
Saran wrap
Phone or computer with speaker AUX port
Materials
Prep time: 10 min
Active time: 5 min
Observation time: 10 min
- Add the conrstarch to the mixing bowl.
1/4 cup
Cornstarch
2. Measure 125 ml of warm water in the liquid measuring cup, then add it to the cornstarch and mix until all the cornstarch is incorporated. If you are using food colouring, add it to the water BEFORE adding the water to the cornstarch.
3. Set the cornstarch and water mixture aside for 5 minutes while you complete the next steps.
4. If your subwoofer/speaker has a fabric cover on it, remove the fabric so the cone element is exposed, then cover the cone with a layer of Saran wrap.
5. Connect the speaker to your computer or phone using an AUX cord. If you are using a Bluetooth speaker, pair your speaker to your phone or computer.
6. Check the consistency of your cornstarch mixture. If you quickly poke the mixture or try to stir it, it should become a solid that doesn’t stick to your finger or stir easily. If you slowly push your finger into the mixture, it should feel like a thick liquid that traps your finger if you try to pull it out quickly. If your mixture doesn’t feel quite right, check the “Observations” section below for tips on troubleshooting.
7. Lay the speaker on its back with the cone facing up then scrape the cornstarch mixture out of the bowl and onto the Saran wrap covering the speaker cone.
8. Play each frequency below and observe how the cornstarch mixture behaves.
30Hz
50Hz
80Hz
Procedure
Questions
1. Why does a cone-shaped speaker need to be used? Why not a speaker with a flat face?
2. What would happen to the cornstarch mixture if you were to drop a ball of it from high up?
When the cornstarch mixture is quickly poked or stirred, it acts as a solid. When an object is slowly pressed into it, it acts like a liquid. If after letting the cornstarch mixture sit for 5 minutes, the cornstarch settles to the bottom of the bowl and there is a layer of water on top, pour off the water and then test the consistency of your mixture as described in step 7. If your mixture feels to dry (ie. never acts like a liquid), slowly add more water.
When the cornstarch mixture is placed on top of the speaker, it spreads out like a liquid. When each of the frequencies is played, the mixture clumps together and changes shape as it bounces around on the surface of the speaker.
Observations
Sound travels through the air as a soundwave with peaks of high pressure and troughs of low pressure. As this wave travels through a medium (ex. air, liquid, etc.) the peaks of the wave will “push” on the particles of the medium. When a trough reaches the same particles, the “pushing” stops. As a result, the medium’s particles will vibrate back and forth, similar to being pushed on a swing.
The rate of this pushing is called the “frequency” of the sound wave. If 30 peaks of a soundwave pass a given point in 1 second, then the frequency of the wave is 30 waves per second, or 30 Hz. If the rate that the sound wave is “pushing” the particles at matches the resonant frequency of the particles (ie. the frequency that they vibrate best at), then the particle’s vibrations will be greater each time a peak pushes it. Again, this is just like being pushed on a swing: there can be equal timing between each push and each can be just as hard as the last, but each time someone pushes you, you move a bit further.
The cornstarch mixture that you created is called “Oobleck”. Oobleck is a type of non-Newtonian fluid. The viscosity (ie. thickness) of a Newtonian fluid will not change when the fluid is acted on by a force, such as pressure from touching (ex. water). The viscosity of a non-Newtonian fluid will change when the fluid is acted on by a force. This is why Oobleck remains more liquid with the gentle compression (ie. pushing force) from a light touch, but instantly becomes thicker and more solid with greater compression from a hard poke.
When the alternating high and low pressures of a sound wave interact with Oobleck, it alternates between being a thinner, less viscous liquid and a thicker, more viscous liquid. This alternating change of viscosity causes the Oobleck to clump together due to its solid properties, but still be able to change shape due to its liquid properties. At the same time, it’s particles are vibrating back and forth from the “pushes” of the sound wave. This vibration causes the Oobleck to bounce around on the speaker.
Explanation
Applications in the Real World
You may not have been familiar with Oobleck prior to this experiment, but you’ve probably already encounter—and tasted—another non-Newtonian fluid: ketchup. Unlike Oobleck, which becomes more viscous when acted on by a force, ketchup becomes less viscous. That’s why tapping, knocking, or shaking the ketchup bottle makes it come out more easily!
As described above, the resonant frequency of a substance is the frequency that it vibrates best at. Try varying the frequency of the sound played to determine the resonant frequency of Oobleck.
Further experiments
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